Narrow band ZIF receivers tend to have slow AGC loop settling times. This is due in part to the closed loop nature of AGC systems and the fact that narrow bandwidth filters tend to add lengthy phase delays near the filter corner frequencies, thereby placing stability limitations on the maximum loop bandwidth of an AGC loop. For narrow band ZIF receivers, AGC settling times in the 4-6 millisecond range are considered exceptional. Notwithstanding, there are several emerging applications where settling times under 2 milliseconds are required. AGC Loops wide enough to theoretically achieve the desired settling times are typically unstable or under-damped and therefore induce excessive ripple in the AGC loop gain control signal. An alternative solution suggests switching to a faster AGC loop during settling and then switching back to the slower loop during normal operation. Unfortunately, when a fast AGC loop is switched to a slower AGC loop, ripple in the gain control signal due to fast loop tracking of the amplitude modulated (AM) signal can leave the control signal at an arbitrarily high level. Additional settling time is required to recover from this overshoot condition. As will be appreciated by those skilled in the art, this additional settling time only operates to defeat the purpose of adapting the fast loop in the first instance.
A solution to these problems is taught in U.S. Pat. No. 5,742,899, titled "Fast Attack Automatic Gain Control (AGC) Loop For Narrow Band Receivers," the disclosure of which is hereby incorporated by reference. The apparatus described in that patent has functioned well in its intended application, but the present communications market has made it desirable to have so-called multimode communication devices. By multimode it is meant that the communication device is capable of communicating using more than one air interface, such as Advanced Mobile Phone Service (AMPS), Global System for Mobile communications (GSM), and so on. For these types of air interfaces it is preferable to use digital control to control the AGC attenuation in the amplifier stage. Thus there is a need for an AGC control that addresses the needs of multimode communication devices.